the definition of economic efficiency that i'm most familiar with is: all possible transactions are made until no one can be made better off without making someone else worse off, which is not exactly like the definitions mentioned in a good summary in wikipedia: http://en.wikipedia.org/wiki/Economic_efficiency

i'm skeptical of framing the input as money. while money looks like you can measure it the same way as mass, length or time, the forces of economics such as subjective judgments of utility do not act on prices the same way that the impersonal forces of newtonian physics act on quantities of mass, length or time. in other words, money is not "cardinal" the way physics phenomena are, but rather "ordinal."

currently, fossil fuel energy is at least 200 times cheaper than human energy, at least in the u.s., so measuring efficiency using costs and prices is liable to be warped. i conclude that great care and attention should thus be given to the goals and methods for achieving them.

so i like to use a definition of efficiency where the output is: clean air and water, healthy food, snug shelter and plenty of sleep, and the input is: energy and resources.